1. Field of the Invention
One embodiment of the present invention relates to a semiconductor device and a display device including the semiconductor device. In particular, one embodiment of the present invention relates to a sequential circuit and a semiconductor device including the sequential circuit, such as a shift register.
2. Description of the Related Art
Recently, a sequential circuit formed using transistors having the same polarity has been actively developed. In particular, Patent Document 1 discloses a sequential circuit that can suppress change in transistor characteristics.
FIG. 17A illustrates a configuration of a conventional sequential circuit. The conventional sequential circuit includes a transistor T13, and transistors T14 and T15 which are connected in parallel. In the conventional sequential circuit, the transistor T13 is turned on and the transistors T14 and T15 are turned off, thereby outputting a first clock signal C1. When the first clock signal C1 is high, a first scan signal Vg1 becomes high (see FIG. 17B). In an odd-numbered frame, the transistor T13 is turned off, the transistor T14 is turned on, and the transistor T15 is turned off, thereby outputting a first supply voltage VSS (see FIG. 17C). In an even-numbered frame, the transistor T13 is turned off, the transistor T14 is turned off, and the transistor T15 is turned on, thereby outputting the first supply voltage VSS (see FIG. 17D). In this manner, the transistor T15 is turned off in an odd-numbered frame and the transistor T14 is turned off in an even-numbered frame, so that change in characteristics of the transistors T14 and T15 is suppressed.